Pretty common condition caused by the monovalve failing, what happens is its (internal) rubber seal tears causing coolant to be on both sides of the valve slowing or stopping its operation. The mono valve is controlled by the climate control system where as you increase the temperature setting, the voltage to the valve solenoid pulses faster and for longer intervals allowing more heated coolant into the heater core. Replace the monovalve core and your heat will be fine, cost is about $25-30. The aux water pump moves coolant through the heater core during idle, at over 1500 engine RPM, the main water pump provides enough flow so the aux isn't necessary. If you turn the key on to the run pos. with the engine off and the climate control set for heat, you should be able to hear the pump running, if not check its plug, take a voltage reading and replace it if there is voltage present but it doesn't run, last time I bought one it was around $80.

Good luck